1. Field of the Invention
The present invention relates to a piezoelectric vibrator, an oscillator, an electronic apparatus and a radio timepiece.
2. Description of the Related Art
For example, in a cellular phone or portable information terminal devices, a piezoelectric vibrator using crystal and so on is often used as a time source, a timing source of a control signal and the like, a reference-signal source and so on. As this type of piezoelectric vibrator, a tuning-fork type piezoelectric vibrator which is airtightly sealed in a package in which a cavity is formed is known.
In the above package, one substrate in which a concave portion is formed is overlapped on the other substrate and both substrates are bonded to each other, thereby forming a cavity by using the concave portion.
The piezoelectric vibrating piece includes a pair of vibrating arm portions arranged side by side in parallel in a width direction and a base portion cantilever-supporting a base-end portion side of the pair of vibrating arm portions, in which the pair of vibrating arm portions vibrates (swings) close to and away from each other in a width direction from the base-end portion side as an original point at a given frequency.
Incidentally, as cellular phone and portable information terminal devices are becoming small in size in recent years, the piezoelectric vibrating piece is also planned to be further small and thin in size. Accordingly, a clearance between the piezoelectric vibrating piece and a mount surface (inner surface of the package) in a thickness direction is becoming extremely narrow (for example, approximately 20 μm to 100 μm). As a result, when an impact due to falling is added to the piezoelectric vibrator, there is a danger that the vibrating arm portions are displaced (bending deformation) and tip portions of the vibrating arm portions as free ends contact the inner surface of the package. In this case, a crack, a chip and so on may occur at the tip portions of the vibrating arm portions due to an impact at the time of contact, which may cause frequency variation or may cause a breakage of the vibrating arm portions themselves in the worst case.
Accordingly, as countermeasures against the above, it is known that, for example, a concave portion for avoiding the touching between the tip portion of vibrating arm portion and a base member is formed in a portion corresponding to the tip portions of the vibrating arm portions on the mount surface as shown in JP-A 2010-119127 (Patent Document 1).
In Patent Document 1, there is disclosed that, even when the vibration arm portion is displaced in the thickness direction due to the impact such as falling, it is easy to avoid the contact between the tip portion of the vibrating arm portion and the base member as a corner portion (opening edge portion) of the concave portion contacts an intermediate portion of the vibrating arm.
However, there are the following problems in the above related art.
That is, when the piezoelectric vibrating piece is mounted on the mount surface of the base member, the piezoelectric vibrating piece is placed with respect to metal bumps, conductive adhesive and so on applied on the mount surface to be fixed thereon. At this time, mounting is performed so that the piezoelectric vibrating piece is arranged in parallel to the mount surface, however, it has been found that the piezoelectric vibrating piece is actually mounted in a state of being inclined to the mount surface in many cases by being affected by a point that the metal bumps and the conductive adhesive are materials with liquidity, variation in mount conditions, a design error of the piezoelectric vibrating piece, a design error of the base member, specifications of a manufacturing device and so on.
The above points will be explained in more detail with reference to FIG. 17 and FIG. 18.
First, in an ideal case in which the piezoelectric vibrating piece is mounted in parallel to the mount surface of the base member, main surfaces 200a of vibrating arm portions 200 are parallel to the mount surface as shown in FIG. 17, therefore, when the vibrating arm portions 200 are displaced due to an external impact and so on, a line contact between a corner portion 201a of a concave portion 201 and the main surfaces 200a of the vibrating arm portions 200 is possible.
However, as the piezoelectric vibrating piece is mounted in a state of being inclined to some degree as described above, particularly, in a state of being inclined in a width direction of the piezoelectric vibrating piece in many cases, the main surfaces 200a of the vibrating arm portions 200 are actually inclined to the mount surface as shown in FIG. 18. In such case, it is difficult to make a line contact between the corner portion 201a of the concave portion 201 and the main surfaces 200a of the vibrating arm portions 200 when the vibrating arm portions 200 are displaced due to the external impact and so on.
Accordingly, it has been found that only edge-line portions 200b of the vibrating arm portions 200 contact the corner portion 201a of the concave portion 201 and that stress is locally acted on the edge-line portions 200b which are fragile in strength. As a result, a crack, a chip and the like occur in the edge-line portions 200b as the stress is concentrated on the edge-line portions 200b, which leads to reduction of strength and vibration characteristics of the piezoelectric vibrating piece. That is, it is difficult to positively prevent occurrence of a crack, a chip and the like in the piezoelectric vibrating piece at the time of receiving the external impact when there is variation in mounting accuracy of the piezoelectric vibrating piece in related art.